Hydrocarbon resources such as petroleum or natural gas have come to be produced by excavation through wells (oil wells or gas wells, also collectively called “wells”) having a porous and permeable subterranean formation. As energy consumption increases, deeper wells are being drilled, reaching depths greater than 9000 m worldwide and greater than 6000 m in Japan. In wells that are continuously excavated, the productive layer is stimulated in order to continuously excavate hydrocarbon resources efficiently from subterranean formations of which permeability has decreased over time and subterranean formations of which permeability has gradually become insufficient. Acid treatment and hydraulic fracturing are known as stimulation methods (Patent Document 1). Acid treatment is a method in which the permeability of the productive layer is increased by injecting an acid such as hydrochloric acid or hydrofluoric acid into the productive layer and dissolving the reaction components of bedrock (carbonates, clay minerals, silicates, and the like). However, various problems that accompany the use of strong acids have been identified, and increased costs, including various countermeasures, have also been pointed out. Thus, perforation for forming pores and hydraulic fracturing for forming fractures in the productive layer using fluid pressure have received attention.
Hydraulic fracturing is a method in which perforations or fractures are generated in the productive layer by fluid pressure such as water pressure (also simply called “hydraulic pressure” hereinafter). Generally, a vertical hole is drilled, and then the vertical hole is curved and a horizontal hole is drilled in a subterranean formation several thousand meters underground. Fracturing fluid is then fed into these boreholes (meaning holes provided for forming a well, also called “downholes”) at high pressure, and fractures and the like are produced by the hydraulic pressure in the deep subterranean productive layer (layer that produces the hydrocarbon resource such as petroleum or natural gas), and the productive layer is thereby stimulated in order to extract and recover the hydrocarbon resource through the fractures and the like. The efficacy of hydraulic fracturing has also been examined for the development of unconventional resources such as shale oil (oil that matures in shale) and shale gas.
Fractures and the like formed by fluid pressure such as water pressure immediately close due to formation pressure when the hydraulic pressure is no longer applied. To prevent a fracture closure, a proppant is included in the fracturing fluid (that is, the well treatment fluid used in fracturing), which is fed into the borehole at high pressure, thereby distributing the proppant in the fracture. Inorganic or organic materials are used as proppants included in fracturing fluid, but silica, alumina, and other inorganic particles have been conventionally used, and sand particles such as 20/40-mesh have been widely used because they are capable of preventing fracture closure in a very deep subterranean environment under high-temperature and high-pressure for a long time.
Various types of water-based, oil-based, and emulsion-based fluids are used as injection well treatment fluids which are fed in at high pressure, such as fracturing fluid. Because the well treatment fluid must have the function of transporting the proppant to the location where the fracture is generated in the borehole, it generally must have a prescribed viscosity, good proppant dispersibility, ease of after-treatment, and low environmental load. Furthermore, fracturing fluid sometimes contains a channelant in order to form flow paths through which shale oil, shale gas, and the like can pass among the proppant. Accordingly, in addition to the proppant, various additives are used in well treatment fluid, such as channelants, gelling agents, antiscale agents, acids for dissolving rock and the like, friction-reducing agents, and the like.
The following method is typically used to produce fractures and perforations by hydraulic pressure in the productive layer of a deep subterranean formation (layer that produces the hydrocarbon resource such a petroleum such as shale oil or natural gas such as shale gas) using fluid fed in at high pressure. Specifically, a prescribed section of a borehole (downhole) drilled and completed in a subterranean formation several thousand meters deep is partially isolated while isolating sequentially from the tip portion of the borehole, and fluid is fed in at high pressure into the isolated section to produce fractures and perforations in the productive layer. Then, the next prescribed section (typically ahead of the preceding section, i.e., a segment closer to the ground surface) is plugged to produce fractures and perforations. After that, this process is repeated until the required isolation and formation of fractures and perforations have been completed.
Stimulation of the productive layer is sometimes also performed again not only for drilling of new wells but for desired sections of boreholes that have already been formed. In this case as well, the operations of borehole isolating, fracturing, and the like are similarly repeated. Additionally, there are also cases where, to perform finishing of the well, the borehole is plugged to isolate fluid from below, and after finishing of the top portions thereof is performed, the isolating is released.
Various methods are known for isolating and fracturing of boreholes, and Patent Documents 2 to 4 disclose plugs that can isolate or fix a borehole (also called a “frac plug,” “bridge plug,” “packer,” or the like). For example, Patent Document 2 discloses a downhole plug for well drilling (also simply called “plug” hereinafter), and specifically discloses a plug comprising a mandrel (main body) having a hollow part in the axial direction, a ring or annular member along the axial direction on the outer circumferential surface orthogonal to the axial direction of the mandrel, a first conical member and slip, a malleable element formed from elastomer, rubber, or the like, a second conical member and slip, and an anti-rotation feature. Isolation of the borehole by a downhole plug for well drilling is performed as follows. Specifically, by moving the mandrel in the axial direction thereof, as the gap between the ring or annular member and the anti-rotation feature gets smaller, the slip contacts the slanted face of the conical member, and by proceeding along the conical member, it expands radially in the outward direction, contacts the inside wall of the borehole, and is fixed in the borehole to seal the borehole, and also, the malleable element deforms by diametric expansion, contacts the inside wall of the borehole, and seals the borehole. The mandrel has a hollow part in the axial direction, and the borehole can be sealed by setting a ball or the like therein.
Patent Document 5 discloses a sleeve system (sometimes called a “frac sleeve”) in which a prescribed number of fracture sleeve pistons (sometimes called “pistons” or “piston plugs”), provided by piercing a passageway through the center part inside a sleeve, are arrayed sequentially such that they can move in the axial direction of the sleeve, and by selectively sealing desired passageways of the piston, the spaces above and below the piston are isolated to form a blocked isolation zone in the borehole. By pressurizing the isolation zone with air or fluid, locations that were weakened in advance are destroyed, and perforations or fractures are generated in the inside walls of the borehole. Specifically, Patent Document 5 describes that sealing of the passageway is performed by a separately proposed ball sealer (also simply called “ball”), and to seal reliably, a ball valve seat is provided at a location that the ball sealer contacts in the passageway provided in the piston. A ball valve seat is also typically called a “ball seat” or simply a “seat,” and it may be the same member as or a different member than the piston body.
Downhole plugs for well drilling are arranged sequentially inside the well until the well is completed, but must be removed at the stage when production of petroleum such as shale oil or natural gas such as shale gas (hereinafter collectively called “petroleum and natural gas” or “petroleum or natural gas”) is begun. Because the plug is typically not designed to be retrievable after use and release of isolation, it is removed by destruction or by making it into small fragments by milling, drill out, or another method, but substantial cost and time are required for milling, drill out, and the like. There are also plugs specially designed to be retrievable after use (retrievable plugs), but since plugs are placed deep underground, substantial cost and time are required to retrieve all of them.
Patent Document 2 describes that metal materials (aluminum, steel, stainless steel, and the like), fibers, wood, composite materials, plastics, and the like are widely exemplified as materials that form plugs, and that composite materials containing a reinforcing material such as carbon fibers, especially polymer composite materials of epoxy resin, phenol resin, and the like, are preferred, and that the mandrel is formed from aluminum or a composite material. On the other hand, Patent Document 2 describes that, in addition to the previously described materials, a material that degrades depending on temperature, pressure, pH (acidic, basic), and the like may be used. However, Patent Document 2 does not disclose whether a material containing a biodegradable material is used for a downhole tool or any part thereof.
Patent Document 3 discloses a disposable downhole tool (meaning a downhole plug or the like) or a member thereof containing a biodegradable material that degrades when exposed to the environment inside a well, and as the biodegradable material, discloses a degradable polymer such as an aliphatic polyester such as polylactic acid. Additionally, Patent Document 3 describes a combination of a tubular body member having an axial-direction flow bore, a packer element assembly comprising an upper sealing element, a center sealing element, and a lower sealing element along the axial direction on the outer circumferential surface orthogonal to the axial direction of the tubular body member, a slip, and a mechanical slip body. Furthermore, Patent Document 3 discloses that fluid flow in only one direction is allowed due to the fact that a ball is set in the flow bore of the cylindrical body part.
Patent Document 4 describes a hydraulic regulating mechanism which performs perforation and fracturing in well drilling for hydrocarbon resource recovery such as a downhole tool such as a bridge plug, comprising a metal-based element that degrades when exposed to the conditions of a downhole, and a swellable member that is bonded with the metal-based element and swells when exposed to the conditions of a downhole. Specifically, as the metal-based element, Patent Document 4 describes a slip and a mandrel formed from a reactive metal selected from aluminum, calcium, and magnesium, or lithium, gallium, indium, or an alloy such as zinc. As the swellable material, Patent Document 4 describes a seal formed from styrene-isoprene block copolymer, polyvinyl alcohol, polylactic acid, or the like. As the swellable material, Patent Document 4 discloses ones that swell approximately 50 to 250% when exposed to salt like sodium chloride, and ones that swell through exposure to hydrocarbons.
Furthermore, in the sleeve system (frac sleeve) described in Patent Document 5, a detachable plug is described, wherein the sleeve and piston can be engaged or released by screwing in or out, using a combination of a removal tool and a detent for receiving the removal tool provided on the top portion of the piston plug, instead of the conventional method using drill out or milling to remove the piston (piston plug) at the stage when production is begun.
Due to increased demand for securement of energy resources and environmental protection, particularly as excavation of unconventional resources expands, excavation conditions are becoming increasingly harsh, such as increased depth. Furthermore, diversification of excavation conditions is advancing, such as the diversification of temperature conditions from approximately 60° C. to approximately 200° C. attendant to diversification of depth. Specifically, various properties are required for the downhole materials used in downhole tools such as frac plugs, bridge plugs, packers, and sleeve systems (frac sleeves). These properties include mechanical strength (tensile characteristics and compression characteristics) to allow the material to be transported to a depth of several thousand meters underground; oil-resistance, water-resistance, and heat-resistance such that mechanical strength and the like are maintained when it comes in contact with the hydrocarbon resource to be recovered in the high-temperature and high-humidity environment of a deep subterranean downhole; seal performance such that isolation can be maintained by high hydraulic pressure when isolating a downhole for perforation and fracturing; and the like. Additionally, the characteristic of being easily removable under the environmental conditions of that well (as described previously, there are diverse environments with regard to temperature conditions and other conditions accompanying diversification of depth) at the stage when a well for hydrocarbon resource recovery has been completed has also come to be required. Downhole tools used in well drilling are arranged sequentially inside the well until the well is completed, and well treatment such as fracturing and perforation are carried out using high-pressure fluid. Then, various sensors, flow paths, and the like are arranged as downhole tool members in order that all well treatments can be completed, the seal can be released, and the next well treatment can be executed repeatedly in sequence. For these sensors, flow paths, and the like, when downhole tools are arranged inside a subterranean borehole, protection is performed by a protecting member or a protective coating (these also qualify as downhole tool members) so that breakage or damage does not occur due to friction, due to contact or collision with other members, or due to the high-pressure fluid used in well treatment. For example, a rubber member such a urethane rubber is used. When the sensors or flow paths are to perform their required functions, the protecting member or protective coating must be removed. Therefore, it has also come to be demanded that the protecting member for a downhole tool which protects the sensor, flow paths, and the like has a protective function for the sensors, flow path, and the like, as well as a function of being easily removable or recoverable.
Thus, there has been a demand for a member for a downhole tool that can reliably perform downhole isolation and the operations of well treatment such as fracturing, and, as necessary, can be easily removed and can easily secure a flow path under a diversity of well environment conditions due to the increasing harshness and diversity of excavation conditions such as increasing depth, and that contributes to reduced expense or shortening of processes.